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GCN Circular 19806

Subject
GRB 160816A: Swift-XRT afterglow detection
Date
2016-08-17T07:10:35Z (8 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU),
S.L. Gibson (U. Leicester), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), A. D'Ai (INAF-IASFPA), A. Maselli  (INAF-IASFPA), A.
Melandri (INAF-OAB) and P.A. Evans (U. Leicester) report on behalf of
the Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Fermi/LAT-detected burst GRB 160816A (Racusin et al. GCN Circ. 19802),
collecting 1.6 ks of Photon Counting (PC) mode data between T0+25.2 ks
and T0+31.8 ks. 

An uncatalogued X-ray source is detected and is above the RASS limit,
and is therefore likely the GRB afterglow.  The position of this source
is RA, Dec=322.4108, +37.1324 which is equivalent to:

RA (J2000): 21:29:38.59
Dec(J2000): +37:07:56.6

with an uncertainty of 3.8 arcsec (radius, 90% confidence).  This
position is 3.8 arcmin from the Fermi/LAT position.  The light curve is
consistent with a constant source of mean count rate 7.8e-02 ct/sec. A
power-law fit gives an index of 1.5 (+1.8, -1.7).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.7 (+0.6, -0.3). The
best-fitting absorption column is  2.97 (+3.55, -0.16) x 10^21 cm^-2,
consistent with the Galactic value of 2.8 x 10^21 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 4.5 x 10^-11 (5.8 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.97 (+3.55, -0.16) x 10^21 cm^-2
Galactic foreground: 2.8 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.7 (+0.6, -0.3)

The results of the XRT-team automatic analysis of the likely afterglow
are at http://www.swift.ac.uk/ToO_GRBs/00020685/index_1.php.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00020685.

This circular is an official product of the Swift-XRT team.
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